It's not different, but that's not what he did. First he passed his hash, which presumably had words as keys and numbers as values, to reverse, which happily treated it as a list and reversed the list, which he assigned to another hash, turning the reversed list back into a hash. For example:
my %hash = ('one' => 1, 'two' => 2, 'three' => 3, 'four-minus-one' =>
my %newhash = reverse %hash;
# %newhash now contains (1 => 'one', 2 => 'two', 3 => 'four-minus-one'
# (1 => 'one', 2 => 'two', 3 => 'three')
See, it turned the original hash into a list and reversed it. Since the last item in the list was a value, that became the first item in the reversed list, becoming a key, with the next-to-last item in the original list (that value's key) becoming its value, and so on. And if any value appeared twice in the original list, it can't appear more than once as a key in the new hash, so all but one of its matching keys will be clobbered. Also, you can't know which key/value pairs will get clobbered, since a hash is unordered by definition.